1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
In the field of optical communications, an optical transmission system using a wavelength-division-multiplexing (WDM) system has been developed to achieve high-capacity transmission. As a pump light source for an optical fiber amplifier used in the WDM system, there is an increasing demand for a pump semiconductor laser in 1480 nm or 980 nm band. It is known that a technology of providing a fiber Bragg grating (FBG) in an optical fiber is very effective in stabilization of the wavelength of a laser light emitted from a semiconductor laser device.
There has been a conventional problem in the semiconductor laser device using the FBG, of temporal instability due to fluctuation of the output power of the emitted laser light.
As a method of mitigating this problem, a method of providing an extra FBG of equal half width of a reflectance-wavelength spectrum and of equal reflectance to those of the other FBG in the optical fiber is proposed. In McGowan and D. Crawford, ThGG42 “Dual Bragg Grating Frequency Stabilization of a 980 nm Diode Laser”, OFC2002 Technical Digest, pp. 671-672, in particular, conditions for obtaining a stable laser output are studied by variously changing an interval between FBGs or a distance from a semiconductor laser to one of the FBGs, which is closer to the semiconductor laser.
If a light output is unstable in a semiconductor laser device using the FBG, an oscillation spectrum is observed as a phenomenon such that the oscillation longitudinal mode of the semiconductor laser temporally switches between a single-mode state and a multi-mode state. To stabilize the light output, it is important to always maintain multi-mode oscillation. Such an oscillation state is called “coherence collapse”. The technology disclosed in the above literature is effective to obtain such a coherence collapse state. However, even if the multi-mode oscillation is maintained, the light output still sometimes fluctuates temporally.
Although temporal fluctuations of the light output tend to occur around a threshold current of the semiconductor laser in general, the stabilization of the light output is required even in a current range closer to the threshold current because of recent improvement in performance of optical fiber amplifiers.